Method of manufacturing carbon fibers

ABSTRACT

Polymeric fibers are heated to elevated temperatures in a bed of heated solid particles, as for instance hollow alumina ballotini. The bed is fluidized with a medium such as air as the fibers contact the heated bed. The method is useful to preoxidize or pyrolyze and optionally to postoxidize polymeric fibers such as polyacrylonitrile, ultimately producing carbon fibers of high strength.

United States Patent Inventors Ian Whitney Wirksworth; John William Johnson, Allestree, Derby, both of England App]. No. 804,639 Filed Mar. 5, 1969 Patented Oct. 26, 1971 Assignee Rolls-Royce Limited Derby, England Priorities Mar. 6, 1968 Great Britain 10819/68;

Oct. 15, 1968, Great Britain, No. 48734/68 METHOD OF MANUFACTURING CARBON FIBERS 16 Claims, 2 Drawing Figs.

US. Cl 23/2094, 8/115.5, 23/2091 Int. Cl C0lb 31/07 [50] Field of Search 23/2091,

[56] References Cited UNITED STATES PATENTS 3,011,981 12/1961 Soltes 252/502 3,285,696 11/1966 Tsunoda.... 23/2091 3,412,062 11/1968 Johnson et a1. 260/37 3,476,703 11/1969 Wadsworth et a1. 260/37 3,508,871 4/1970 Cory 23/2091 Primary Examiner Edward J. Meros Attorney-Cushman, Darby & Cushman ABSTRACT: Polymeric fibers are heated to elevated temperatures in a bed of heated solid particles, as for instance hollow alumina ballotini. The bed is fluidized with a medium such as air as the fibers contact the heated bed. The method is useful to preoxidize or pyrolyze and optionally to postoxidize polymeric fibers such as polyacrylonitrile, ultimately producing carbon fibers of high strength.

METHOD OF MANUFACTURING CARBON FIBERS This invention relates to a method of manufacturing carbon fibers.

Throughout this specification the term "fluidizing medium" is to be understood to mean the gas or other medium used to fluidize the particles, spheres or other solid objects which comprise the bed.

According to the present invention a method of manufacturing carbon fibers by the pyrolysis of a polymeric fiber, comprising at least one step in which the fibers is heat-treated in a fluidized bed.

The fluidizing medium of said bed preferably reacts with the fiber.

Heaters may be provided in the bed to achieve and maintain the desired temperature in the bed; these heaters preferably comprise electrical-resistance heaters.

The starting material preferably comprises a polyacrylonitrile fiber, although other fibers such as a polyamide may be used.

Said step may comprise a preoxidation step in which the polymeric fiber is preoxidized prior to pyrolysis.

The preoxidation may be carried out at a temperature in the region of 200-400 C., but preferably between 200 and 350 C.

In an alternative embodiment said step comprises an etching treatment after pyrolysis of said fiber. Said etching step may be carried out with the bed at a temperature of between 400 and 500 C.

The invention will now be particularly described merely by way of example with reference to the accompanying drawings in which:

F 1G. 1 is a diagrammatic view of apparatus for carrying out the method of the present invention, and

FIG. 2 is a diagrammatic view of apparatus for carrying out a second embodiment of the method of the invention.

Referring to FIG. 1, the starting fiber, which in this case is 1% denier polyacrylonitrile fiber, is wound from a supply drum in the form of a sheet of fiber onto a drive unit 11 which comprises a plurality of rollers 12 some of which are driven and which are in frictional engagement with the fiber. The drive unit 11 pulls the fiber from the supply drum l0 and transports it through an inlet duct 30 into a fluidized bed 13 for preoxidation treatment.

The fluidized bed 13 comprises an insulated container 14 having a perforated bottom 15, the lower surface of which is supplied with compressed air from the duct 16. The space above the perforated bottom 15 is filled with ballotini which may be glass or hollow alumina, the latter being less demanding of fluidizing medium, up to a level shown at 17. lmmersed in the ballotini are electrical-resistance heaters 18 and a plurality of rollers 19.

It will be appreciated that the compressed air flowing through the duct 16 and through the perforated bottom 15 causes the ballotini to be fluidized in a well-known fashion. The heaters 18 are used to heat the ballotini to a required temperature which in the present instance is 220 C. although it would be possible to use a temperature in the range 200 to 400 C., the lesser range 200-350 being preferred.

The fiber emerging from the drive unit 11 passes over the rollers 19 in a tortuous path whereby the total path length in the fluidized bed is made to be relatively large. Thus in this particular instance the speed of the drive unit and the path length in the fluidized bed may be arranged to give the fibers a treatment time of approximately 7 hours in the bed.

While the fiber is immersed in the fluidized bed it will be appreciated that it will be caused to remain at the temperature of the bed, that is, 220 C., and the compressed air used to fluidize the bed will provide an oxidizing atmosphere around the fiber. In this way the fiber will be subjected to a preoxidizing treatment equivalent to that which could be carried out in a normal air furnace but because of the high capacity of the bed and the rapid transfer rate taking place in it there will be very little danger of the temperature of the fibers departing from the required temperature.

Thus although the preoxidizing reaction is slightly exothermic and in certain conditions could run away if the temperature is allowed locally to rise above a set limit, the fluidized bed will virtually preclude any such happening with its consequent catastrophic results.

Thus a single bed may be used with a reduced treatment time; if a lengthy treatment (e.g. 7 hours) is used it may be necessary to use a plurality of beds in series to avoid fluidization problems in the lee of the fiber sheet. If such a plurality of beds is used they may be arranged to give a temperature gradient overall, which may improve oxidation performance.

The inlet duct 30 will provide a temperature gradient from ambient temperature at its top end to the bed temperature at its lower end; thus the fiber is gradually heated to the bed temperature rather than being plunged into the high-temperature bed. This temperature gradient will dry the incoming fiber, preventing a lowering of the melting point, and will prevent the formation of gaseous pockets formed by the too rapid volatilization of included solids.

When the fiber leaves the last of the rollers 19 it is fed into a second drive unit 20 which comprises a plurality of rollers 21 similar to those of the drive unit 11. The second unit may be arranged to provide, in conjunction with the first unit 11, some stretch of the fiber within the bed. Thus a stretch of l4 percent has been found to efl'ectively replace the tension normally generated within the fibers when an air preoxidation is used. The drive unit 20 passes preoxidized fiber into a pyrolysis furnace 22 which is not described in detail but is such as to enable the fiber to be heated to a temperature of l,000 C. over a time period of some one-half to 3 hours and then to cool naturally to room temperature. On leaving the furnace 22 the fiber has had the majority of its constituents, other than carbon, driven off and is fairly pure carbon fiber having reasonably good structural properties for use as a reinforcement in a matrix.

On leaving the furnace 22 the carbon fiber is wound onto a takeup spool 23 via a roller 24 and may then be used as a reinforcement. In some typical experimental examples of treatment, a fluidized bed was used in which the fibers were treated by winding a layer on a frame and immersing under the surface of the ballotini. ln one instance 12 fibers were chosen from a batch treated using the conditions outlined above and were found to have a mean breaking stress of 282Xl0 p.s.i., and a mean modulus of 27.6Xl0 p.s.i. In a second example the fibers were wound in two layers on the frame in an attempt to provoke a run-away of the reaction. The preoxidation temperature was raised to 250 C. and the treatment time reduced to half an hour. In this case the 12 samples tested gave mean breaking stresses of 209 l0 p.s.i., and the mean modulus of 20.2Xl0 p.s.i. It will be noted that these results are somewhat less attractive than those in the first example but they still represent values which make the fiber useful reinforcement for matrix materials, while it will be appreciated that the double layer of fiber probably affected the results.

In FIG. 2 apparatus is shown which is suitable for the postoxidation of fiber. in this case fiber, which is 1% denier carbon fiber produced by the pyrolysis of polyacrylonitrile fiber, is wound from a supply drum 40 in sheet form onto a drive unit 41 which comprises a plurality of rollers 42 some of which are driven and which are in frictional engagement with the fiber. The drive unit 41 pulls the fiber from the supply drum 40 and transports it into a fluidized bed 43.

The fluidized bed 43 comprises a container 44 having a perforated bottom 45, the lower face of which is supplied with compressed air from the duct 46. The space above the per forated bottom 45 is filled with hollow alumina ballotini up to a level shown at 47. lmmersed in the ballotini are electrical-resistance heaters 48 and a plurality of rollers 49.

Due to the high ambient temperature the rollers 49 are preferably mounted for rotation on air bearings (not shown), which are supplied with compressed air from the duct 46 to maintain their operation. Alternatively they may be arranged to be situated in the cold zone of the bed below the heaters.

It will be appreciated that the compressed air flowing through the duct 46 and through the perforated bottom 45 causes the ballotini to be fluidized in a well-known fashion. The heaters 46 are used to heat the ballotini and air to a required temperature which in the present instance is 450 C. although it would be possible to use a temperature in the range 400' to 500 C., the lesser range 400-450 being preferred.

The fiber'emerging from the drive unit 41 passes over the rollers 49 in a tortuous path whereby the total path length in the fluidized bed is made to be relatively large. Thus in this particular instance the speed of the drive unit and the path length in the fluidized bed may be arranged to give the fibers a treatment time of approximately -15 minutes in the bed.

Again while the fiber is immersed in the fluidized bed, it will be appreciated that it will be caused to remain at the temperature of the bed, that is 450 C., and the compressed air used to fluidize the bed will provide an oxidizing atmosphere around the fiber. in this way the carbon fiber will be subjected to a surface etching treatment equivalent to that which could be carried out in a normal air furnace but, because of the high heat capacity of the bed, and the rapid transfer rate taking place in it, there will be very little danger of the temperature of the fibers department from the required temperature.

Thus although the surface-etching reaction is again slightly exothermic and, in certain conditions, could run away if the temperature is allowed locally to rise above a set limit, the fluidized bed will virtually preclude any such happening. The temperature control possible when carrying out these processes in a fluidized bed leads to the possibility of raising the treatment temperature to a limit which would be dangerous if carried out in a normal air furnace. In this way the treatment time may well be considerably reduced. Thus a single bed may be used with a reduced treatment time; if a lengthy treatment is used it may be necessary to use a plurality of beds in series to avoid fluidization problems in the lee of the fiber sheet. If such a plurality of beds is used they may be arranged to give a temperature gradient overall, which may improve etching performance.

When the fiber leaves the last of the rollers 49 it is fed into a second drive unit 50 which comprises a plurality of rollers 51 similar to those of the drive unit 41. The drive unit 50 passes the carbon fiber onto a takeup spool 53 via a roller 54 and may then be used as a reinforcement.

It will be noted that in the above example the process was applied to a sheet of fiber passing through the fluidized bed. It will be appreciated that this sheet could be replaced by a tow or a plurality of tows which typically have of the order of 10 fibers. In this case the conditions of the fluidized bed become even more advantageous since, when there are a plurality of fibers side by side, it becomes very easy to produce local hotspots in a nonnal air-furnace treatment. Using the fluidized bed these hotspots are virtually eliminated as described above and the method of the present invention, therefore, makes etching of such a plurality of tows quite feasible.

It will be noted that, although the carbon fibers described were made from polyacrylonitrile as a starting fiber, it would be possible to use other suitable copolymers of polyacrylonitrile or a difierent polymer, such for instance as a cellulose, polyamide etc.

The ballotini referred to above could be replaced-by balls or spheres of other materials; thus if the fluidized bed is to be used at very high temperatures, silica sand particles may be used.

it will be appreciated that although the invention was described above using air as the fluidizing medium for the bed, it would of course be possible to use other fluids. Thus oxygen could be used, or for the part-etching treatment, air or oxygen having an additive of a reaction-slowing chemical such as chlorine. In this latter case a considerably higher temperature, even as much as l,000 C., could be used in the fluidized bed, and in fact, there will be a considerable range of temperatures and times within which the etching treatment can take place.

The etching treatment described above has a dual purpose. As described in our prior British application No. 58492/66 it etches away surface flaws which otherwise might reduce the strength of the carbon fibers. Also it has been discovered that the etching process reacts on the fiber surface in such a way as to enable considerably better adhesion between thefibers and other material which forms the matrix, thereby considerably improving the interlarninar shear strength of the composite material.

The method described above is highly advantageous in that it enables precise control of the treatment temperature, enabling what is a mildly exothermic reaction to be accurately controlled without any risk of explosion and also enabling treatment temperatures to be increased while remaining reasonably safe.

It will also be noted that the present invention uses the fluidizing medium of the bed as one of the reagents in the chemical reaction involved, thereby enabling it to serve a dual purpose and avoiding the necessity for separate supplies of reagent and fluidizing medium.

As an alternative to the immersion type of fluidized bed described above, it may be preferable to use a surface treatment bed in which the fibers are arranged to lie along the surface of a long shallow bed. This method allows the use of a bed of simple design with low air pressure, and might prove advantageous for long treatment times.

What we claim is:

1. In the manufacture of carbon fibers by the pyrolysis of a polymeric fiber, the improvement comprising conducting at least one step of the heat treatment of said fiber in intimate contact with a bed of heated solid particles retained in a container and supplied with a fluidizing medium to fluidize said particles.

2. The method of claim 1 wherein the improvement comprises heating said solid particles, introducing to said container a fluidizing medium to fluidize said heated solid particles and passing said fiber through said bed of fluidized heated solid particles.

3. In a method of manufacturing carbon fibers by the pyrolysis of a polymeric fiber comprising the successive steps of:

a. preoxidizing said fiber by heating in an oxidizing atmosphere at elevated temperatures; and

b. pyrolyzing said fiber thus produced by heating the oxidized fiber at a temperature of up to about at least 1 ,000 C. for a period of time until substantially all of the fibers constituents except carbon are removed; the improvement wherein at least one of steps (a) and (b) is conducted in contact with a bed of heated solid particles retained in a container and supplied with a fluidizing medium to fluidize said particles.

4. The method of claim 3 comprising the additional step of: c. postoxidizing said pyrolyzed fiber in an oxidizing atmosphere for a time sufficient to permit surface etching of said fiber; the improvement wherein at least one of steps (a), (b) and (c) is conducted in said bed of heated particles.

5. The method of claim 3 wherein said solid particles are grains or ballotini.

6. The method of claim 5 wherein said solid particles are hollow alumina ballotini.

7. The method of claim 3 wherein step (a) is conducted at a temperature of about 200 to 400 C.

8. The method of claim 3 wherein step (a) is conducted from one-half to 3 hours.

9. The method of claim 3 wherein the fiber is stretched while in said bed.

10. The method of claim 3 wherein step (a) is conducted in said bed of heated particles and said fibers enter the bed by way of a duct which is supplied with hot gas from the bed, so that a temperature gradient is established in said duct.

11. The method of claim 3 wherein said solid particles are heated by electrical resistance.

12. The method of claim 3 wherein step (a) is conducted in said bed of heated particles and said fluidizing medium is air.

a bed of heated particles and the fluidizing medium comprises chlorine.

16. The method of claim 4 wherein said polymeric fiber is polyacrylonitrile. 

2. The method of claim 1 wherein the improvement comprises heating said solid particles, introducing to said container a fluidizing medium to fluidize said heated solid particles and passing said fiber through said bed of fluidized heated solid particles.
 3. In a method of manufacturing carbon fibers by the pyrolysis of a polymeric fiber comprising the successive steps of: a. preoxidizing said fiber by heating in an oxidizing atmosphere at elevated temperatures; and b. pyrolyzing said fiber thus produced by heating the oxidized fiber at a temperature of up to about at least 1,000* C. for a period of time until substantially all of the fibers constituents except carbon are removed; the improvement wherein at least one of steps (a) and (b) is conducted in contact with a bed of heated solid particles retained in a container and supplied with a fluidizing medium to fluidize said particles.
 4. The method of claim 3 comprising the additional step of: c. postoxidizing said pyrolyzed fiber in an oxidizing atmosphere for a time sufficient to permit surface etching of said fiber; the improvement wherein at least one of steps (a), (b) and (c) is conducted in said bed of heated particles.
 5. The method of claim 3 wherein said solid particles are grains or ballotini.
 6. The method of claim 5 wherein said solid particles are hollow alumina ballotini.
 7. The method of claim 3 wherein step (a) is conducted at a temperature of about 200* to 400* C.
 8. The method of claim 3 wherein step (a) is conducted from one-half to 3 hours.
 9. The method of claim 3 wherein the fiber is stretched while in said bed.
 10. The method of claim 3 wherein step (a) is conducted in said bed of heated particles and said fibers enter the bed by way of a duct which is supplied with hot gas from the bed, so that a temperature gradient is established in said duct.
 11. The method of claim 3 wherein said solid particles are heated by electrical resistance.
 12. The method of claim 3 wherein step (a) is conducted in said bed of heated particles and said fluidizing medium is air.
 13. The method of claim 3 wherein said polymeric fiber is polyacrylonitrile.
 14. The method of claim 4 wherein step (c) is conducted at a temperature of about 400* to 500* C.
 15. The method of claim 4 wherein step (c) is conducted in a bed of heated particles and the fluidizing medium comprises chlorine.
 16. The method of claim 4 wherein said polymeric fiber is polyacrylonitrile. 